Surface-treated multi-layered polymer film

ABSTRACT

A sealable polymeric film structure comprising (a) a core layer including a thermoplastic polymer, the core layer having a machine side and a product side that is on a side of the film opposite from the machine side; and (b) a sealable skin layer positioned on the product side of the core layer, the sealable skin layer including a thermoplastic polymer, wherein an exterior surface of the sealable skin layer has a receding Cahn value of at least about 0.48; and wherein in some heat—sealable embodiments, the sealable skin layer sealed to itself has a MST of from about 175° F. to about 220° F., with a jaw pressure of 20 psi for 0.75 seconds and a minimum acceptable seal strength of 200 g/in at 175° F., on a Wrap-Ade™ Crimp Sealer Model J or K.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application claims benefit of priority to Provisional U.S.Application No. 60/613,157 filed Sep. 24, 2004 and is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to polymer films, such as are useful forpackaging films. More particularly it relates to heat sealable,biaxially-oriented film structures that are surface treated on thesealing surface. In preferred embodiments, the film is a multilayerfilm.

BACKGROUND OF THE INVENTION

Multilayer polymeric films are commonly used for packaging products suchas snack foods, including candies, potato chips, cookies, ice cream barsand dairy products. The versatility and compositional variation ofpolymer films facilitate usefulness in a variety of packagingapplications. A wide array of film compositions are available today dueto custom tailoring a multitude of variables related to polymercomposition, additives, and method of preparation of the film to suit aparticular product application and service environment. Seemingly smallchanges in composition or properties often determine whether or not afilm will perform suitably. For example, although polymer films havemany desirable properties, untreated or uncoated films may possesscertain inherent disadvantages, such as relatively poor waterwetability, poor printability, poor barrier properties and/or blockingwith adjacent film layers.

Frequently, a primary concern in designing multiple-layer films forpackaging is to ensure that the film can be processed, prepared andutilized on high speed converting and form-fill-and-seal (“FFS”)machinery. Vertical FFS packaging apparatus operate by unwindingcontinuous film from bulk rolls, pulling the film over a forming collar,followed by forming pouches or bags by sealing lower and side portionsof the package, filling the package, and finally sealing the upperportion to close the package. Horizontal FFS packaging apparatus operateby unwinding continuous film from bulk, aligning the film along ahorizontal track, positioning the product or container, such as an itemor box, on the film in a determined position and orientation, followedby forming and bending the film around the product or container, eithersnuggly or loosely, to form a package, and thereafter sealing thepackage closed.

Therefore, in many packaging operations, the film must have sufficientflexibility and machinability to undergo mechanical folding from aroll-fed or flat orientation to a folded condition and be subjected to asealing function which is part of high-speed packaging apparatus. Thehigher the packaging speed, the more difficult this operation becomes.In addition to aesthetic and optical qualities, the ability of the filmto perform reliably on high speed equipment, including unrolling,processing, folding and sealing equipment, are typically primaryobjectives in film composition design. Line speed is dictated by anumber of factors, including friction with machinery, blockingtendencies, sealability, and packaged product interaction with the film.For many such packaging operations, the product-film interaction factoris not a big consideration. However, for some applications, such asfoods, powders, grease-bearing products, and moist or wet products, thisfactor may become of quite significant importance with respect to theinteraction over an extended period of time. An additional considerationfor the product-film interaction factor is the interaction preciselyduring packaging operations, as this factor may significantly controlthe ability to increase packaging operation speed.

The performance of a film embodiment, during packaging operations, mayvary with changes in environmental conditions and can adversely impactproperties on one or both sides of a film. For example, in a dairyenvironment where frozen ice cream novelties and frozen water-based popsmay be packaged, temperature and humidity conditions may vary widely andinconsistently at various regions within the facility, and within thevicinity of packaging operations, where the packaging equipment andprocesses are located. For example, the packaging film may be utilizedin relatively dry conditions for a period of time and then under morehumid and even wet conditions at another time, when the packagingequipment and/or adjacent packaging equipment is washed with water,which may occur several times per day. Also, the composition of theproducts on a line may vary widely, such as from sugar-based tosugar-free variety of a product.

In a humid or wet environment, undesirable friction characteristics mayadversely affect the performance of either or both sides of the film.Friction between packaging equipment surfaces and the film, and/orbetween the product and the film may cause packaging imperfections andimpede high-speed processing. For example, frozen pops or ice cream barsthat are made from a syrup, cream or sugar-based solution may readilyadhere and “tack-up” as the product engages the product side of thefilm, remaining in the proper position and orientation for the packagingprocess, while a sugar-free version of the product may slide or reorienton the film during packaging operations, resulting in either operationalproblems and slowing down the packaging line to accommodate the productvariations. A misaligned product may create increased product waste,film waste, machine damage and down-time, due to the product gettingcaught or chopped in the seals during the packaging process.

On the opposite side of the film, the machine side, the film must beable to slide over a variety of stationary, rotating, metal, nonmetal,heated, and/or refrigerated surfaces, while maintaining a stablecoefficient of friction and good slip characteristics relative to all ofthe surfaces. As relative humidity increases and condensation forms onequipment, the moisture may adhere to the film creating an excessivestaccato movement of the film as the film repeatedly sticks and thenreleases from the wet machine surfaces. This staccato movementfrequently results in inconsistent package length, film jams, tears, andexaggerated product misalignment problems on the opposing, product sideof the film.

There exists in the art, the need for a sealable film with improvedprocessability, machinability, and reliability over a range oftemperature, relative humidity and dampness conditions, ranging fromrelatively dry to a substantially wet environment, without adverselyimpacting other desirable film parameters, such as film sealability,moisture barrier properties, interaction with the packaged product, andoptical and appearance properties.

SUMMARY OF THE INVENTION

This invention provides a film that has functionally usefulfriction-related properties on both sides of the film to facilitateimproved machinability and processability in a relatively high humidityor moist environment, to facilitate improved packaging speed. One sideof the film may offer improved processing and packaging machineryinteraction while the opposing product side of the film providesadvantages for interaction between the film and certain products, suchas aqueous-based products. The film exhibits acceptable sealability,desired appearance and optical properties, is substantiallynon-blocking, and may be useful with moisture-comprising products and inrelatively humid conditions, such as with freshly baked confectioneries,dairy products, and water-based products. The film may be suitable foruse in packaging relatively low-moisture-content products, such as dryfoods, and for use in dry environments. The film may be particularlywell suited for use with horizontal form, fill and seal applications(HFFS), vertical, form, fill, and seal applications (VFFS), overwrapapplications, pouches, bags, and for use in multilane HFFS packagingapplications. The multi-layer film can be used as a coextruded web, oras a component of a laminated film structure. More specifically, theinvention provides a film structure that includes a polymer core layerand a heat-sealable polymer skin layer, with the exterior surface of theskin layer possessing sufficient surface-energy to facilitateadvantageous film interaction with various products.

A key aspect of this invention i s that the exterior surface of thesealable skin layer, (typically the product side surface, that is, thesurface of the sealable skin layer opposite the core layer side of thesealable skin layer) possesses relatively high energy or is treated toincrease the surface energy thereof. If treated to increase the energy,the film is preferably only treated to a limited extent to preventdegradation of the sealability of the sealable skin layer and to preventtreating the opposing side of the film. The relatively high energy orsurface treatment may render the product side exterior surface of thefilm substantially hydrophilic in nature. Additionally, the exteriorsurface of the film on the side of the core layer opposite the sealablelayer, the machine side surface of the film, is either not treated toincrease surface energy, or is treated but the surface energy propertiesare attenuated, such as by printing or coating, such that the surface issubstantially low-energy or hydrophobic in nature, as compared to theproduct side exterior surface. Films according to the present inventioncomprise the combination of both; (i) the treated or relativelyhigher-energy, substantially hydrophilic product side surfaceproperties, and (ii) the untreated or relatively low-energy,substantially hydrophobic, machine side surface properties. However,there may be applications for some embodiments w here the machine sidesurface may be lightly treated, such as for printability, and then atleast partially obscured to prevent the machine side from being toohydrophilic.

DETAILED DESCRIPTION OF THE INVENTION

For clarity purposes, the films described herein may be referred to ashaving a product side and a machine side, the machine side beingopposite the product side. The product side of the film may be definedas the side or surface of the film or layers of the film that may beconsidered on the inside of a package created using the film, which isthe side that typically faces toward or comes in contact with a productthat may be packaged using the film. The machine side of the film may bedefined as the outside or side or surface of the film or layers of thefilm, that face opposite from the product side and may typically engagethe film feeding and handling components of the packaging machinery andsupport package printing and graphics.

Preferred embodiments are multilayer, coextruded polymer filmstructures, typically comprising a core layer and two skin layers,though one or more tie layers may be present between the core layer anda skin layer. The term “core layer” as used herein refers to the onlylayer of a monolayered film or the thickest layer of a multilayeredfilm. The core layer of a multilayer structure typically will be theinnermost or more centrally positioned layer of the film structure withrespect to the other, more external layer(s) on one or each side of thecore layer. It is understood that when a layer is referred to as being“directly on” another layer, no intervening layer(s) is/are present. Onthe other hand, when a layer is referred to as being “on” another layer,intervening layers may or may not be present. The core layer includes aproduct side and a machine side.

It is commonly understood in the art that polymer films have two major,outer-most surfaces, each being an exterior surface and each on anopposite side of the film from the other. The term “exterior” as usedherein refers to such major surfaces, with one such surface on a“product side” of the film and an opposing exterior surface of the filmpositioned on a “machine side” of the film.

The polymer film includes a core layer and the core layer may be clearor made opaque such as by cavitation. The core layer comprises apolymeric matrix, preferably containing olefin polymers and morepreferably containing a propylene polymer. Suitable preferred core layerpolyolefin materials include polypropylene, polyethylene, polybutene,and copolymers and blends thereof. The term “propylene polymer” as usedherein includes homopolymers as well as copolymers of propylene, whereina copolymer not only includes polymers of propylene and another monomer,but also terpolymers, etc. However, in many preferred embodiments thepropylene polymer is a propylene homopolymer. An isotactic polypropylenecontaining at least 80% by weight of isotactic polypropylene may beparticularly preferred in many embodiments. The propylene polymer of thecore layer preferably has an isotacticity ranging from about 80% to100%, preferably greater than 85%, most preferably about 95% to 96%, asmeasured by ¹³C NMR spectroscopy using meso pentads. A mixture ofisotactic propylene polymers may be used. When a mixture is used,preferably the mixture comprises at least two propylene polymers havingdifferent m-pentads. Preferably, the difference between m-pentads is atleast 1%. Furthermore, the propylene polymer of the core layerpreferably has a melt flow index ranging from about 2 to about 10 g/10minutes, most preferably from about 3 to about 6 g/10 minutes, asmeasured according to ASTM D1238 at 190° C. under a load of 5 lbs. Insome embodiments, the core layer may also comprise a blend or copolymercontaining propylene and another C₄-C₁₀ olefin.

Commercially available and exemplary propylene polymers that aresuitable for the c ore layer of many embodiments may include PP 3371, anisotactic propylene homopolymer sold by Atofina Petrochemicals (Houston,Tex.), and PP 4712, an isotactic propylene homopolymer from ExxonMobilChemical Company (Houston, Tex.).

Though many preferred film embodiments are relatively clear ortransparent, in some embodiments the core layer may be voided orcavitated to create an opaque film. Cavitation may be effected such asby conventional cavitating methods using void-inducing particles thatare incompatible with the polymer. Suitable conventional void-inducingparticles may include organic particles such as polybutyleneterephthalate (“PBT”) or nylon, or inorganic particles such as calciumcarbonate. Cavitation may also be introduced by beta-cavitation, whichincludes creating beta-form crystals of polypropylene and converting atleast some of the beta-crystals to alpha-form polypropylene crystals,thereby creating a small void remaining at the situs after theconversion. Preferred beta-cavitated embodiments of the core layer mayalso comprise a beta-crystalline nucleating agent to induce creation ofthe beta-form polypropylene crystals. Substantially any beta-crystalnucleating agent (“beta nucleating agent” or “beta nucleator”) may beused. The core layer may also include pigments, dies, or other hazingand/or coloring agents.

The film includes a sealable skin layer that is preferably coextrudedwith the core layer and is positioned on the product side of the corelayer. The sealable layer may be defined to include a first side on theside of the sealable layer that is on the product side of the corelayer, and a second side opposite the first side, the second sideproviding an exterior surface for the film on the product side of thecore layer. The sealable skin layer preferably comprises a polymermaterial that i s heat sealable and provides the seal propertiesnecessary for the desired application. Exemplary polymer materials forthe sealant layer include homo-, co-, and terpolymers containing olefinpolymers, such as polypropylene, polyethylene, polybutene, andpreferably those that are heat sealable. Other sealant layer materialsmay include a polylactic acid and preferably propylene-ethylenecopolymers or propylene-ethylene-butylene terpolymers orethylene-propylene-butylene terpolymers.

In still other embodiments, one or more product side polymerictie-layers may be positioned between the product side of the core layerand the sealable skin layer. Suitable product side tie layer polymersmay include one or more of the polymers such as may be suitable for thecore and/or sealable skin layer.

In some film embodiments, the sealable skin layer may also contain aprocessing additive, such as an anti-block agent, e.g., Tospearl™particulates, a Mitsubishi Polymers product. For example, a particulateslip/antiblock system may comprise non-migratory crosslinkedhydrocarbyl-substituted polysiloxane particles, and/or silica particles.

A novel and beneficial feature of a film according to this invention isthat the product side exterior surface of the sealable skin layer eitherinherently possess a relatively high surface energy or is treated toincrease the surface energy or wetting tension of that surface, ascompared to the untreated wetting tension of such surface. However, iftreatment is required to attain the desired surface energy, the amountof treatment should be carefully limited or controlled such that thesurface is not over treated to the point that the treatment destroys orunacceptably reduces the sealability of the sealable skin layer. Acontrolled treatment level is also desirable so that other performancecharacteristics, such as optical and barrier characteristics are notimpaired and so that the opposing exterior surface of the film is notinadvertently treated. If the inherent surface energy of the sealantlayer polymer is insufficient to provide the desired wetability, theexterior surface of the film on the product side of the core layer maybe treated by any of the known treatment techniques, depending upon thedesired film use and desired degree of treatment, such as by corona,flame, or plasma treatment, with corona treatment preferred for manyapplications.

A key benefit realized from increased wetting tension and surface energyon the product side exterior surface of the sealable film is improvedadhesivity or tackup with respect to interaction between the filmsurface and a product being packaged. The increased or relatively highsurface energy may improve the packaging-line performance of the filmfor certain products, such as moisture-bearing products, over a widevariety of moisture-related conditions, as compared to the interactionbetween such product and an untreated film surface. The more energizedproduct side exterior surface may afford improved performance of thefilm in packaging aqueous-based products and/or provide improvedpackaging use in a humid or wet environment. The relatively high surfaceenergy may increase the molecular attraction between the film surfaceand the product. This increased energy may translate into an increasedsurface wetting tension, rendering the film surface more hydrophilic andattractive to products containing polar components, such as frozennovelty or ice-pop treats. The benefit may be most apparent if thepackaged product is one that has polar tendencies, such as water. Theinventor has learned that during high-speed packaging of aqueous-basedor moist products, the products more readily adhere or tack-up better onthe relatively high-energy film surface than on an untreated orotherwise lower-surface-energy surface. The improved molecularattraction may help the product adhere to and tack-up on the filmsurface, without easily sliding about, reorienting, or becomingmisaligned, as can happen with products on film surfaces that tend to bemore charge-neutral or somewhat hydrophobic. The molecular attractionfrom the energized surface may also provide packaging line operationalbenefit when the product is frozen and does not substantially wet out onthe film, such as with frozen ice-pop treats. It has also been learnedthat the treated surface also may not adversely impact the performanceof the film with respect to packaging relatively dry and/or non-polarproducts or when used in a relatively less humid environment. Thereby,the film may be useful in a variety of packaging operations and over awide humidity range.

The benefit of improved film-product interaction results from the filmhaving a relatively energized product side skin layer, exterior surface.Film surface energy is a solid surface characteristic associated withthe molecular forces of the interaction of that surface with anothermaterial. Surface energy is the true film characteristic that isdesirable to measure, but it may not easily be measured directly.Therefore, this property may be characterized by measuring one of twosubstitute properties, namely “wetting tension” or “contact angle.” Bothof these measurements involve observing the behavior of liquids placedon the films' surface. Surface tension is the force that exists betweena liquid and the atmosphere it is in. For example, in atmospheric air, adrop of water will tend to “bead up” on some solid surfaces, such as anewly waxed vehicle hood. It is primarily the surface tension existingbetween the water and the air that produces this effect. The drop ofwater can spread, or wet-out, on a solid surface if the solid surfacehas molecular forces (surface energy) high enough to overcome thewater/air surface tension and draw the water relatively flat onto thesolid surface, such as a treated film surface. Wetting tension is themaximum liquid surface tension that will spread, rather than bead up, onthe film surface. It is a measurable property that estimates a film'ssurface energy. ASTM D 2578 provides a test procedure for determiningwetting tension by applying different test solutions of increasingsurface tensions until one is found that just spreads (wets) the filmsurface. Units are dynes/cm. However, although the wetting tension testmay be relatively quick and simple to perform, the results tend to varywidely such that repeatability may be difficult to obtain.

Measurement of contact angle may be a more preferred method ofcharacterizing the surface energy of a film. One method of measuringcontact angle is by using a Cahn Model DCA-300, DCA-312, or DCA-315contact angle measurement instrument to measure the dynamic contactangle (DCA). Unlike the ASTM contact angle test that applies a singlewater droplet, the Cahn test immerses a prepared film sample into abeaker of distilled water at a constant rate of speed, while sensitivebalance records the “advancing” wetting force. The instrument thenreverses and pulls the sample from the water, recording the “receding”wetting force. The Cahn DCA software contains well establishedcalculations to convert the measurements into advancing and recedingcontact angle values. The measurement value that is reported is the“Cahn value,” which is calculated as the cosine of the receding anglethat forms between the measuring fluid and the subject surface as thefilm is pulled from the liquid.

The receding contact angle is frequently recognized and accepted as thebest indicator of surface energy adhesion properties. The receding angleis the angle at which a drop of liquid forms when being pulled away fromthe film surface, and is the angle that is of most interest. The higherthe receding DCA Cahn value, the lower the angle, the higher the surfaceenergy, and the better the water-film adhesion. A low receding angle isconsistent with a relatively water-wet surface, demonstrating acorresponding, relatively high surface energy. Relatively highersurface-energy surfaces reflect an increased Cahn value, which mayapproach 1.00. The angle and cosine value is calculated by the Cahn DCAmeasuring equipment and a receding angle of about 30 degrees may exhibita Cahn value of about 0.87, a receding angle of about 45 degrees mayhave a Cahn value of about 0.71, an angle of about 60 degrees may have aCahn value of about 0.50, and a relatively lower surface energy surfacemay have an angle approaching 90 degrees and a corresponding Cahn valuethat is relatively low or approaching zero. Some relatively low Cahnvalues, such as may be measured on relatively low energy, hydrophobicsurfaces may even demonstrate a reported Cahn value that is less thanzero.

Over time, the surface energy of some film embodiments may dissipate orlose some of the energy that was present immediately after treatment. Afilm surface may exhibit a particular surface energy shortly aftertreatment and a reduced level of energy a few weeks later. For purposesherein, the Cahn values provided refer to surface energies or Cahnvalues of films that are aged at least two weeks after either surfacetreatment or after production if not treated.

The product side exterior surface of films according the presentinvention may either inherently possess a surface energy level or may betreated to possess a surface energy level demonstrating a receding DCACahn value of at least about 0.48. Though the exact amount of surfaceenergy required for suitable performance of a film in any particularpackaging operation may vary according to humidity, product composition,temperature and shape, packaging line speed and numerous othervariables, it has been determined that for packaging of products thatare water-based or possess a substantial moisture content, for manypreferred film embodiments and applications, the multilayer film shouldinherently possesses or be treated to possess a receding Cahn value ofat least 0.48.

As for a desired or target upper Cahn value limit, if the receding DCACahn value of at least 0.48 represents surface energy that is inherentlypresent in the film at the level desired without treating the surface,then no upper limit Cahn value may be relevant and the upper limit mayeffectively approach 1.00. However, from an operational standpoint, ifthe product side of the film must be treated to obtain the desired ortarget Cahn value, then it is preferred that the target Cahn valueshould not exceed about 0.89 to prevent over-treatment of the film. Formany preferred embodiments, the product surface may possess or may betreated to possess a Cahn value of from at least about 0.60 to about0.87. More preferably, the product surface may possess or may be treatedto have a Cahn value of from at least about 0.64 to about 0.87. In stillother preferred embodiments, the product surface may possess or may betreated to have a Cahn value of from at least about 0.65 to about 0.78.Thereby, the film according to this invention may possess sufficientsurface energy on the product surface to facilitate improved reliabilityand line-speed during packaging operations of water-based products, overa wide variety of moisture-related environmental conditions and productmoisture content, as compared to film having a non-treated or relativelylower exterior surface energy.

In heat-sealable film embodiments according to the present invention,the heat-sealable skin layer should comprise a polymer composition thatprovides a minimum seal temperature (“MST”) within a range that permitsthe film to be commercially useful for its intended purpose. Theacceptable MST range is from about 175° F. to about 220° F., with a jawpressure of 20 psi for 0.75 seconds, on a Wrap-Ade™ model J or K crimpsealer, with vertically serrated crimp jaws, wherein the product side ofthe skin layer sealed to itself. Seal strength testing is performed on aSuter™ tester that pulls the seal a part at 12 in/min. MST seal strengthis 200 g/in, and is determined at 175° F. seal temperature.

For films requiring surface treatment to energize the product sideexterior surface of the film, it is preferred that the amount oftreatment is limited or controlled so that the treatment will notadversely affect other desirable film properties such as sealability,optical, and/or aesthetic properties. For example, many polymer basedsealable skin layers, such as those comprising propylene and/or ethylenehomo-, co-, and ter-polymers, may require surface treating to possesssufficient surface energy to perform acceptably at high packaging speed.It has been learned that by providing a controlled or limited treatmentlevel on the film product surface when such treatment is required thatthe surface may retain acceptable sealing properties.

As for the exterior-most surface of the film on the machine side of thecore layer, in many film embodiments according to this invention, suchsurface or layer is not surface treated. Thereby, the machine sideexterior surface may be relatively hydrophobic as compared to theproduct side of the film. It is desirable for the machine side of thefilm to be relatively non-water-wet, such that water tends to “bead-up”and not wet-out on the exterior surface. There may be some applicationswhere the desired functionality and/or polymer composition on themachine side surface of the film requires a limited amount of surfacetreatment to accommodate another need, such as when printing on suchsurface with an aqueous-based ink. Otherwise, it may be advantageous inmost embodiments not to surface treat the machine side surface of thefilm substrate, thereby maintaining the surface energy in a relativelyreduced state as compared to the product side of the film. The reducedsurface energy may minimize the adverse impact on processability causedby an accumulation and wetting out of moisture, such as by condensationor water overspray, upon the metal parts of the processing equipment. Apolymeric film surface that exhibits increase surface energy may alsoexhibit an increased molecular attraction between the film, water, andpackaging machine components, creating a processing problem. When themoisture on the equipment or in a damp environment comes in contact witha treated machine side surface, the water tends to wet-out or spreadacross a greater areal extent of the film web than it would on anuntreated, non-water-wet film, which may result in increased friction,reduced slip, and increase molecular attraction. Such occurrence canproduce mild to severe processing problems, including plasticallydeforming or tearing the film during packaging operations. As such, toprevent such problems it is preferred that the machine side surface ofthe sealable film not be treated or should be designed to possess aninherently low surface energy. Not wishing to be bound by such theory,the inventor believes that the relatively low surface energy maydemonstrate suppressed molecular attraction between the film surface,the moisture, and the machine components such that the friction betweenthe film and the wet equipment surfaces is reduced as compared to thefriction between a treated machine side film surface and wet machinesurfaces. It is recognized, however, there may be applications where themachine side surface can be lightly treated with acceptable filmprocessability and machinability.

In films according to the present invention, the receding DCA Cahn valueof the exterior-most surface on the machine side of the film should notexceed a Cahn value of 0.20, preferably not greater than about 0.17, andmore preferably not greater than about 0.15. For some embodiments, theCahn value measurements may even be slightly negative values.

In some embodiments where some level of treatment of the machine sideexterior surface is required, the most exterior machine side surface maybe treated and then partially coated or printed, to attenuate orpartially obscure any molecular attraction effect created by the surfacetreatment. For example, a machine side exterior surface of the film maybe treated such as by plasma treatment and then printed with an ink,pattern-coated with a coating, or coated with a non-polar coating,whereby the ink or coating covers at least about ten (10) percent of thesurface area of the machine side exterior surface of the film. The inkor coating may mitigate the effect of the surface treatment such thatthe receding DCA Cahn value of the surface after treating and coating orprinting is not greater than about 0.20, preferably not greater thanabout 0.17, and more preferably not greater than about 0.15. Also, insome embodiments the machine side skin layer may comprise an antiblockagent or other additives therein to not only attenuate the effect of asurface treatment, but also to prevent blocking and further reducemachine side friction.

In a basic form, films according to the present invention may compriseonly a polymeric core layer and a polymeric sealant skin layer. In suchembodiments, the machine side of the core layer may provide the machineside exterior surface of the film. In many preferred embodiments, one ormore polymeric layers may be provided on the machine side of the corelayer. Thereby, such exterior surface of the one or more polymericlayers may provide the machine side, exterior surface of the film.

The machine side skin layer comprises a thermoplastic polymer resin orcombination of resins as may be suitable for the intended processingconditions, printing, or use of the film, preferably an olefin polymer.For example, the machine side skin layer may comprise polypropylene,polyethylene, polybutylene, polyesters, and blends thereof. In onepreferred embodiment, the machine side skin layer comprises primarilypolyethylene, such as HDPE, MDPE, LLDPE, LDPE, VLDPE, copolymers ofethylene, such as ethylene-propylene copolymer, and blends thereof.Other embodiments may comprise a high crystallinity polypropylene.

Machine side tie layers may also be present between the machine sideskin layer and the core layer. Such machine side tie layers may comprisea thermoplastic polymer, preferably a polyolefin or polyester polymer.The machine side tie layer may include opacifying or coloring agents orpigments, such as whitening agents, including titanium dioxide.

Though not critical, the overall thickness of a film according to thisinvention may typically range from about 5 microns to about 60 microns,and may preferably range from about 10 microns to about 50 microns, andeven more preferably from about 30 microns to about 45 microns.

The invention further relates to a method of making a sealable film thatis suitable for packaging operation use with moist or water-bearingproducts and for use in a humid or wet environment. The methodpreferably comprises steps for making a heat sealable film embodiment.However, other film embodiments may be prepared that are sealable otherthan by heat seal, such as by application of a cold glue adhesive to theexterior surface of the sealable skin layer. Suitable cold glueadhesives may include water or solvent based adhesives that require somedrying to achieve a full strength bond, or an adhesive such as a rubber,latex, or other “tacky” type of adhesive or cement. In the broadestsense, films according to the present invention may be sealable on theinside surface or product side surface of the film. Preferredembodiments may be heat-sealable through incorporation of a coextrudedheat-sealable skin layer, such as a terpolymer olefin sealable skinlayer.

One preferred method for preparing a film comprises the steps of; (a)coextruding a sealable polymeric film structure including a product sideand a machine side, the film structure comprising (i) a core layerincluding a thermoplastic polymer, the core layer having a machine sideand a product side on an opposite side of the core layer from themachine side, and (ii) a sealable skin layer positioned on the productside of the core layer, the sealable skin layer comprising athermoplastic polymer, wherein an exterior surface on the product sideof the sealable polymeric film has a receding Cahn value of at leastabout 0.48; and (b) orienting the coextruded sealable film structure inat least one direction. Preferably, the film i s biaxially oriented,though some embodiments may be uniaxially oriented. Also, the Cahn valueis measured after orientation and treatment, and after the film has agedfor at least two weeks after treatment.

The method may further comprise the step of surface treating the productside exterior surface of the skin layer with one of corona treatment,flame treatment, and plasma treatment. Surface treating may commonly berequired to provide the outer surface of the sealable skin layer withthe desired surface energy.

Though methods according to this invention may be useful to preparefilms having a Cahn value of at least 0.48, wherein such films may besuitable for many applications and are within the scope of thisinvention, preferably the methods are used to prepare films wherein theexterior surface of the sealable skin layer has a Cahn value of fromabout 0.6 to about 0.87, or more preferably a Cahn value of from about0.65 to about 0.78. Though some film embodiments may be prepared havingCahn values greater than the upper limits prescribed herein, the upperlimits may be preferred as useful guidelines in that values in excess ofthose prescribed may possess more energy than is necessary to obtainsuitable film packaging performance. However, as many film embodimentsmay be prepared with coextruded heat-sealable olefin or other polymerresins as the sealable skin layer, typically such sealable layer willrequire surface treatment to provide sufficient surface energy to renderthe surface suitable for use in the improved applications as describedherein. As surface treating can destroy or reduce the heat-sealabilitycharacteristics of a heat-sealable skin layer if too much treatmentenergy is used, it is preferred that the application of surface energybe limited to only an amount of surface energy necessary to providesuitable packaging performance during a period of time after treatmentdate. Thereby, it is recommended that the film only be treated topossess a limited or target level of surface energy to avoid such overtreating. Thus, the upper limits prescribed herein may serve asguidelines for selecting an appropriate treating level and method.

The methods of this invention may also be utilized to prepare a heatsealable film as may be used in many preferred embodiments andapplications. The method may comprise the step of coextruding aheat-sealable, skin layer. Such heat-sealable skin layer may be sealedto itself, such as with a fin seal or end seal, or sealed to theopposing machine side of the film such as with a lap seal. When theproduct side is sealed to itself, such as a fin or end seal, the filmpreferably has a MST of from about 175° F. to about 220° F., with a jawpressure of 20 psi for 0.75 seconds and a minimum acceptable sealstrength of 200 g/in at 175° F., on a Wrap-Ade™ Crimp Sealer Model J orK. Alternatively, the seal may be effected by applying one of a coldglue adhesive and a pressure sensitive adhesive to the product sideexterior surface of the sealable skin layer and/or to both the productside sealable skin layer and the machine side of the film. In suchalternative embodiments, it may not be necessary that the sealable skinlayer comprise a resin that would render the skin layer heat sealable.For example, the sealable skin layer may comprise an isotacticpolypropylene or polyethylene having a relatively high crystallinity.

Preferred methods may also comprise coextruding a second skin layer onthe machine side of the core layer. Thereby, the film has skin layers oneach side of the core layer. The method may also comprise coextrudingtie layers, such as a product side tie layer between the core layer andthe sealable skin layer, and/or a machine side tie layer between thecore layer and the second skin layer.

Though some methods of producing films according to this invention donot include surface treating the machine side exterior surface, someembodiments may benefit from surface treatment of that side to provideother film functionalities. For example, the machine side surface mayrequire printing. The methods for preparing the film may include thestep of applying a minimial amount of surface treatment to facilitateproper ink wetting and adhesion. However, if the aerial extent of inkcoverage exceeds an application-determinative threshold, preferably atleast about ten percent of the film surface area for some embodiments,such coverage may sufficiently dampen or attenuate the surface energycreated by the surface treatment such that the net receding DSC Cahnvalue is not greater than 0.20, after coating or printing. A similarattenuation effect may be obtained by pattern coating the exteriorsurface with a coating formulation, such as an acrylic coating on atleast about ten percent of the surface area.

The methods according to this invention may also comprise a method ofpackaging a product in a sealable polymeric film structure. Suitablemethods for packaging a product may comprise the steps of; (a) drawing asealable polymeric film structure through a packaging machine from aninlet side of the packaging machine to an outlet side of the packagingmachine, the sealable polymeric film structure including a product sideand a machine side, the multilayer film comprising (i) a core layercomprising a thermoplastic polymer, the core layer having a machine sideand a product side, and (ii) a sealable skin layer comprising athermoplastic polymer, wherein an exterior surface of the sealable skinlayer has a receding Cahn value of at least about 0.48; (b) positioninga product on the product side exterior surface of the sealable skinlayer; (c) contacting a first portion of the product surface of the filmwith another portion of the product surface of the film to form apackage cavity and to bond the first portion with the second portionover the contacted area, wherein the product is positioned within atleast a portion of the package cavity; and (d) applying at least one ofheat and pressure to at least a portion of the seal area to bond thefirst portion of the sealable skin layer to the second portion of thesealable skin layer and encase at least a portion of the product withinthe product space. Contacting a first portion of the product surfacewith another portion of the product surface means that the product sidesealable skin layer is sealed to itself, such as with a fin seal or withan end seal. Though fin seals may be preferred, it is also within thescope of this invention that the product side of the film may be sealedto the machine side of the multilayer film, such that the sealable skinis sealed to either the second skin layer or the core layer on themachine side of the core layer. The step of applying either or both ofheat and pressure to at least a portion of the seal area means that thefilm may be either heat sealable and/or it may be sealable by meansother than heat, such as with a cold glue adhesive or cement. The termproduct may be defined broadly to include substantially any item, good,or article that may be packaged at least partially, within a filmpackage created with films according to this invention.

A product to be packaged by the film may be placed or positioned on theproduct side of the film, such as by feeding from a conveyor or otherproduct transport mechanism. In some preferred applications thisoperation may be performed on a horizontal FFS machine. The product maybe substantially any article or item for packaging, wherein the productis fed onto the film, however, aqueous-based products may realize thehighest benefit from packaging in film according to this invention,particularly an edible product such as a frozen ice-pop treat, ice creambar, yogurt-based product, or substantially any other sugar-based orsugar-free treat or novelty bar.

The step of contacting a first portion of the product side surface ofthe film with another portion of the product side surface of the film toform a package cavity may typically involve folding opposing edgemargins of the film to engage each other, forming a pocket or spatialregion that may confine or envelope at least a portion of the product.The opposing edge margins may be engaged with each other to facilitatefin sealing, lap sealing, or end sealing, preferably fin sealing.Preferably heat, but alternatively a contact adhesive or cold-glueadhesive may be applied to the engaged portions oft he product sidemargin(s) to cause the first edge margin to seal to the second edgemargin. When heat is used to create the seal, the sealable skin layerresin type and thickness will determine the appropriate amount of heat,pressure, and duration of time to the contacted area to form asubstantially hermetic seal over the contacted area. The package mayalso be sealed in like fashion on each end, transversely from the firstseal, to create the end seals and confine the product within the productcavity. TABLE I Optical Gauge, mil = 1.4 Film I: Film II: Poly Gauge,mil = 1.2 Receding Receding Yield, in²/lb = 24,500 Cahn Cahn Density =0.81 Value Value Before After Sealant Sealant Layer Layer TreatmentTreatment Outside (Machine Side) 0.9465 0.1503 HDPE + antiblock SpartechA27527 (PE) TOTAL = 4.8 ga. Corona Treated Untreated Print  4.0% PrintLayer Layer PP Homo + TiO₂ Exxon 4612 TOTAL = 18.0 ga. 15.0% PP Homo +PBT Exxon 4612 TOTAL = 73.8 ga. 61.5% PP Homo Exxon 4612 TOTAL = 180 ga.15.0% Terpolymer w/ Chisso 7791 TOTAL = 5.4 ga. Untreated Sealant CoronaTreated Tospearl  4.5% Layer Sealant Layer Inside (Product Side) −0.04930.6916

Table I demonstrates two similar film compositions, Film I and Film II,with the fourth column from the left displaying the receding DCA CahnValue (cosine of the contact angle) of the comparative, prior art Film Ithat was treated on the outside for printability, and the fifth columndemonstrating the Cahn value of Film II prepared according to thepresent invention. The film embodiments are identical except that theprior art Film I is treated on the machine side outer surface and is nottreated on the product side or sealable skin layer. The illustrativeFilm II embodiment is treated on the product side and reflects asubstantially energized product side surface as compared to theuntreated surface. The outside or machine side of Film II, however, isuntreated and reflects a substantially reduced surface energy ascompared to the prior art film.

Film I was used in packaging sugar-free novelty ice-pop treats at apackaging machine rate of 20.5 lifts per minute, resulting in an averagewaste of over 3300 lbs of product over a 20 hour production run.However, the inventive Film II and methods discussed herein demonstratedimproved 20 hour production runs with product wastes of less than 1300lbs per run, with a packaging machine operating speed of 21.6 lifts perminute. Film II represents a product waste reduction of 61% with acorresponding 5.4 percent increase in packaging line operation rate,translating into substantial savings and line operating efficiency.Additionally, due to the untreated machine side exterior surface, FilmII also demonstrated a reduction in splits, tears, and down time due tofilm transport problems in the packaging machinery.

1. A method of preparing a heat-sealable polymeric film structureincluding a product side and a machine side, the method comprising thesteps of: (a) coextruding: i) a core layer including a thermoplasticpolymer, the core layer having a machine side and a product sideopposite from the machine side; and ii) a heat-sealable skin layerpositioned on the product side of the core layer, the heat-sealable skinlayer including a thermoplastic polymer; wherein the product sideexterior surface of the coextruded film structure has a receding Cahnvalue of at least about 0.48; wherein the sealable skin layer whensealed to itself has a MST of from about 175° F. to about 220° F., witha jaw pressure of 20 psi for 0.75 seconds and a minimum acceptable sealstrength of 200 g/in at 175° F., on a Wrap-Ade™ Crimp Sealer Model J orK; (b) orienting the coextruded film structure in at least onedirection; and (c) surface treating the product side exterior surface ofthe film with one of corona treatment, flame treatment, and plasmatreatment.
 2. The method of preparing a heat-sealable film according toclaim 1, wherein the product side exterior surface of the film has aCahn value of from about 0.6 to about 0.87.
 3. The method of preparing aheat-sealable film according to claim 1, wherein the product sideexterior surface of the film has a Cahn value of from about 0.65 toabout 0.78.
 4. The method of preparing a heat-sealable film according toclaim 1, further comprising the step of: coextruding a second skin layeron the machine side of the core layer, the second skin layer comprisinga thermoplastic polymer.
 5. The method of preparing a heat-sealable filmaccording to claim 1, further comprising the step of: coextruding aproduct side tie layer comprising a thermoplastic polymer, the productside tie layer positioned between the core layer and the sealable skinlayer.
 6. The method of preparing a heat-sealable film according toclaim 1, further comprising the step of: coextruding a machine side tielayer comprising a thermoplastic polymer, the machine side tie layerpositioned between the core layer and the second skin layer.
 7. Themethod of preparing a heat-sealable film according to claim 1, whereinthe exterior surface of the polymeric film structure on the machine sideof the film has a receding Cahn value of not greater than about 0.20. 8.The method of preparing a heat-sealable film according to claim 1,wherein the exterior surface of the polymeric film structure on themachine side of the film comprises a receding Cahn value of not greaterthan about 0.17 dynes/cm.
 9. The method of preparing a heat-sealablefilm according to claim 1, wherein the core layer further comprises acavitating agent.
 10. The method of preparing a heat-sealable filmaccording to claim 1, wherein the thermoplastic polymer of the corelayer comprises at least one of a polyolefin, polyester, and polylacticacid.
 11. The method of preparing a sealable film according to claim 1,wherein the thermoplastic polymer of the sealable layer furthercomprises a polymer selected from the group consisting of polyester,polylactic acid, polypropylene, polyethylene, polybutylene, copolymersor terpolymers of C₂-C₄₀ olefins, copolymers or terpolymers of ethyleneand C₃-C₄₀ olefins, copolymers or terpolymers of propylene and C₂-C₄₀olefins, and copolymers or terpolymers of ethylene, propylene, andbutene.
 12. The method of preparing a sealable film according to claim4, wherein the thermoplastic polymer of the second skin layer comprisesat least one of a polyolefin, polyester, and a polylactic acid.
 13. Themethod of preparing a heat-sealable film according to claim 1, whereinan exterior surface on the machine side of the film has a Cahn value ofnot greater than about 0.20.
 14. The method of preparing a heat-sealablefilm according to claim 1, wherein the exterior surface on the machineside of the film has a Cahn value not greater than about
 0. 17.
 15. Amethod of packaging a product within a heat-sealable polymeric film, themethod comprising the steps of: (a) drawing a heat-sealable polymericfilm structure through a packaging machine from an inlet side of thepackaging machine to an outlet side of the packaging machine, theheat-sealable polymeric film structure including a product side and amachine side, the multilayer film comprising (i) a core layer comprisinga thermoplastic polymer, the core layer having a machine side and aproduct side, and (ii) a heat-sealable skin layer comprising athermoplastic polymer, wherein an exterior surface of the heat-sealableskin layer has a receding Cahn value of at least about 0.48, and whereinthe sealable skin layer when sealed to itself has a MST of from about175° F. to about 220° F., with a jaw pressure of 20 psi for 0.75 secondsand a minimum acceptable seal strength of 200 g/in at 175° F., on aWrap-Ade™ Crimp Sealer Model J or K, wherein the product side exteriorsurface of the film is surface treated with one of corona treatment,flame treatment, and plasma treatment; and (b) positioning a product onthe product side of the heat-sealable multilayer film; (c) engaging afirst exterior portion of the product surface of the heat-sealable skinlayer with one of (i) a second exterior portion of the product surfaceand (ii) the machine side exterior surface of the film, to form a sealarea over the engaged portions of the heat-sealable skin layer andcreate a product space with the film; and (d) applying heat to at leasta portion of the seal area to bond the heat-sealable skin layer to theengaged exterior portion machine side to encase at least a portion ofthe product within the product space.
 16. The method of preparing aheat-sealable film according to claim 15, further comprising the stepof: surface treating the exterior surface of the polymeric filmstructure on the machine side of the film with one of flame treatment,corona treatment, and plasma treatment; and applying one of a printingink and a coating composition on at least about 10% of the area of theexterior surface on the machine side of the film.
 17. The methodaccording to claim 15, wherein the exterior surface of the heat-sealableskin layer has a Cahn value of from at least about 0.60 to about 0.87.18. The method according to claim 15, wherein the exterior surface ofthe heat-sealable skin layer has a Cahn value of from about 0.65 toabout 0.78.
 19. The method according to claim 15, wherein the core layerfurther comprises a cavitating agent.
 20. The method of claim 15,wherein the product comprises one of water, an aqueous-based product, ordairy-based product.
 21. The method of claim 15, wherein the water,aqueous-based, or dairy-based product is substantially frozen.
 22. Themethod of claim 21, wherein the aqueous-based or dairy-based productcomprises at least one of an ice cream treat, a sugar-based treat, asugar-free treat, and a yogurt treat.
 23. A packaged product encased ina heat-sealable film structure, comprising: a product; and a packagingfilm at least partially encasing the product, the packaging filmcomprising a product side and a machine side; (a) a core layer includinga thermoplastic polymer, the core layer having a machine side and aproduct side; and (b) a heat-sealable skin layer positioned on theproduct side of the film, the heat-sealable skin layer including athermoplastic polymer, wherein an exterior surface of the sealable skinlayer has a receding Cahn value of at least about 0.6 to about 0.85, andwherein the heat-sealable skin layer sealed to itself has a MST of fromabout 175° F. to about 220° F., with a jaw pressure of 20 psi for 0.75seconds and a minimum acceptable seal strength of 200 g/in at 175° F.,on a Wrap-Ade™ Crimp Sealer Model J or K.
 24. The packaged product ofclaim 23, wherein the product comprises one of a water based product anda dairy based product.
 25. The packaged product of claim 23, wherein theproduct is substantially frozen.
 26. The packaged product of claim 23,wherein the product comprises one of an ice cream treat, a sugar-basedtreat, a sugar-free treat, and a yogurt-based treat.